1. Field of the Invention
The present invention relates to read-channel signal processing in media storage systems. More particularly, the present invention relates to a method and system for improved read-channel equalization through integration of servo information.
2. Description of Related Art
Data/media storage systems such as hard disk drives (HDDs), compact disks (CDs and CD-ROMs), digital video disks (DVDs), magneto-optical disks, etc., read and write data using magnetic or optical transfer techniques. In the case of magnetic media the process of reading the data involves measuring the magnetic field generated by the recorded data. In the optical media case the process of reading the data involves measuring the optical reflection properties of the recorded data as the data is lit by a laser beam.
Regardless of the technology used to read the data, once the data is read the measured signal is converted into an electrical signal (read signal). The electrical/read signal is then processed to infer the value of the data that was read (detected symbols). This is referred to as read-channel signal processing.
FIG. 1 illustrates a prior art read channel signal processing procedure. Data is stored in the media storage system as binary data 110. The data is accessed using either the magnetic or optical media processes. In FIG. 1, read-channel 120 represents the modulation process where the magnetic or optical flux is written on the medium and the magnetic or optical signal is measured. The read-channel 120 also represents the conversion of the measured magnetic or optical signal into a filtered and sampled electrical signal referred to herein as the read signal 130.
Next, the read signal 130 is processed to infer the value of the data that was read i.e., detected symbols 150. The read-channel processing scheme illustrated in FIG. 1 for inferring the value of the data that was read to arrive at the detected symbols 150 is referred to as Partial Response Maximum Likelihood (PRML) detection 140. PRML detection 140 incorporates a linear partial response equalizer 145 followed by a Viterbi detector 146. Other prior art methods for read-channel processing include Decision Feedback Equalizer (DFE) and Finite Delay Tree Search (FDTS). Such read-channel processes work in similar fashion to the PRML processes and are not discussed in detail herein.
Such read-channel signal processing systems suffer from Inter Symbol Interference (ISI) and noise that makes it difficult for the system to detect each written symbol. Thus, the read-channel processing systems have a corresponding error rate associated with the fact that the detected symbols 150 do not always match the values of the binary data 110 that was written. To decrease the error rate associated with read channel signal processing systems, such systems use models of the read channel to model the dynamic relationship between the actual value of the recorded data and the corresponding measured value of the electrical signal (read signal). The accuracy of these models greatly influences the Bit Error Rate (BER) of the read process. The BER is the ratio of the number of data bits whose inferred value was incorrect over the total number of data bits that were read and processed.
FIG. 2 illustrates a model used in prior art read-channel processing systems. Read channel model (model) 200 typically is a linear model 240 with additive white noise 220. The concepts and processes of modeling are known in the art and are not discussed in detail herein. Model 200 and the processes of model 200 would be used within the read channel 120 illustrated in FIG. 1. In FIG. 2, model 200 represents the assumption that the read signal 230 is produced by passing the binary data 210 through a linear filter 240 and then adding white noise 220.
In prior art media storage systems read-channel signal processing is performed separately from servo processing. Consequently, in prior art read-channel signal processing the only source of error considered is the additive white noise discussed above. By separating the read-channel signal processing from servo processing, the read-channel equalization does not take into account the servo error of the read-head (i.e., actuator head) during processing of the read data. The servo error is the error between the actual position of the head and the desired position of the head on the media storage device. As track pitch is reduced to increase density, the tracking (or position) errors become a significant portion of the track pitch. In systems with reduced track pitch, during real-time operation the actual value of the servo error fluctuates widely due to controller design limitations and external disturbances. The position of the actuator head during a read and/or write operation will affect the value of the data read and/or written.
The positioning and motion of the magnetic or optical heads that read and/or write the data are controlled using sophisticated feedforward and feedback control methods (control methods). The main objective of these control methods is to minimize the servo error and improve the data access time of the system. The servo error, as stated earlier, is the error between the actual position of the head and the desired position of the head on the media storage device. The data access time is the amount of time that passes from the moment the command for reading and/or writing the data is issued to the moment that the data is actually read and/or written.
FIG. 3 illustrates an example of servo error, or positioning error on a rotating media storage device, for example a disk drive. It should be noted that servo error occurs on other media storage devices and that the example of a rotating media storage device such as a disk drive is merely meant to be illustrative and not limiting. It should also be noted that the systems described herein exchange the information on the servo error or positioning error as signals and thus the discussions herein may refer to the servo error or positioning error as the servo error signal or positioning error signal (PES), respectively.
Illustrated in FIG. 3 is an enlarged version of a xe2x80x9ctrackxe2x80x9d 310 on a storage disk. In approximately the center of track 310 is a dashed line 320 which represents the data stored on track 310. Actuator head 330 is illustrated in FIG. 3 as being located directly above the center of track 310 where the data 320 is stored and is also located on a servo wedge 350.
A servo wedge is like a xe2x80x9cmarkingxe2x80x9d on a disk (usually placed there at the time of manufacture) that delineates position on a disk. For example, a servo wedge in a magnetic recording media has a stronger magnetic field than other regions of the magnetic disk so as to delineate position. In the magnetic disk example, when a system senses the stronger magnetic field (i.e., the actuator head passes over a servo wedge) the position of the actuator head can be measured (or xe2x80x9csampledxe2x80x9d) by the system. Based upon the signal detected at the servo wedge the position of the actuator head within the track can also be determined. In other words, the system is able to detect if the actuator head is on the center of the track or is off the center of the track. If the actuator head is off center, the system will also be able to determine which direction from center and by approximately how much the actuator head is off center. When a servo wedge passes under the actuator head, this occurrence is referred to as a xe2x80x9cservo burstxe2x80x9d.
It should be noted that the only time the position of the actuator head can be measured is at the point of a servo burst, and such measurements are referred to herein as xe2x80x9csampled dataxe2x80x9d. Based upon the sampled data the system can determine the position error (PES) of the actuator head.
In FIG. 3, servo bursts are represented as squares having an xe2x80x9cxxe2x80x9d thereon. The actuator head during read and/or write does not necessarily remain directly above the stored data 320 but instead fluctuates. For example, the path of the actuator head (positions 330-339) in FIG. 3 is illustrated as having an almost sinusoidal function as the head follows the track 310. It should be noted that the path of the actuator head may vary from system to system and that the path illustrated in FIG. 3 is meant merely to be illustrative and not limiting.
When the actuator head is not at the center of the track there is a positioning error 360 (servo error) that represents the distance the head is from the center of the track. This servo error 360 contributes to read channel errors. The further the actuator head is from the center of the track the greater the chance for error in reading and/or writing the stored data. As the servo error becomes larger, the ability of these prior art media storage systems to accurately read the data diminishes, and the BER increases. Additionally, as track pitch is reduced, and bits are packed more closely on the track, BER increases due to the combined effects of servo position error and intersymbol interference.
In other words, although seemingly unrelated, the servo error control system has a direct impact on the read-channel signal processing system of a media storage system. This is due to the fact that the read-channel model used for data processing is greatly influenced by the servo error. In particular, the magnetic or optical signal generated by the actuator head as it passes over the location of the data becomes weaker as the servo error becomes larger. Thus, the servo error plays a major role in the process of read-channel equalization, where the effects of the read-channel on the measured signals are removed or equalized before a decision is made about the value of the data (detected symbols).
However, most media storage systems have a separate design for the servo control system from the read-channel signal processing system. Without linking the two together, the read signal equalizer does not have access to the valuable information about head position which would enable the equalizer to improve the BER.
Because the prior art read-channel signal processing systems do not take into account the servo error there are several disadvantages. One disadvantage is the fact that in order to ensure that the BER on such read-channel systems will remain within pre-specified bounds, the performance requirements on the servo control loop become unnecessarily conservative, leading to increased data access times and decreased track density.
Another disadvantage is that, due to servo control limitations, the power in the servo error signal cannot be arbitrarily reduced. Since all the power from the servo error signal is perceived as noise in these prior art read-channel equalizers, the performance of the read-channel equalizers in terms of the BER is unnecessarily low. Consequently, this leads to lower linear bit density on the track.
What is needed is a method and/or system that integrates servo error estimation with read-channel equalization to improve the linear bit density and track density of a media storage system.
A method and system for servo error integration in read-channel equalization are described. Servo error is integrated into a read-channel equalizer.
Other features and advantages of the present invention will be apparent from the accompanying drawings, and from the detailed description, which follow below.